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Fails your build if code quality thresholds are not met.

Discipline will set you free.


gem install cane
cane --abc-glob '{lib,spec}/**/*.rb' --abc-max 15

Your main build task should run this, probably via bundle exec. It will have a non-zero exit code if any quality checks fail. Also, a report:

> cane

Methods exceeded maximum allowed ABC complexity (2):

  lib/cane.rb  Cane#sample    23
  lib/cane.rb  Cane#sample_2  17

Lines violated style requirements (2):

  lib/cane.rb:20   Line length >80
  lib/cane.rb:42   Trailing whitespace

Classes are not documented (1):
  lib/cane:3  SomeClass

Customize behaviour with a wealth of options:

> cane --help
Usage: cane [options]

Default options are loaded from a .cane file in the current directory.

    -r, --require FILE               Load a Ruby file containing user-defined checks
    -c, --check CLASS                Use the given user-defined check

        --abc-glob GLOB              Glob to run ABC metrics over (default: {app,lib}/**/*.rb)
        --abc-max VALUE              Ignore methods under this complexity (default: 15)
        --abc-exclude METHOD         Exclude method from analysis (eg. Foo::Bar#method)
        --no-abc                     Disable ABC checking

        --style-glob GLOB            Glob to run style checks over (default: {app,lib,spec}/**/*.rb)
        --style-measure VALUE        Max line length (default: 80)
        --style-exclude FILE         Exclude file from style checking
        --no-style                   Disable style checking

        --doc-glob GLOB              Glob to run doc checks over (default: {app,lib}/**/*.rb)
        --no-doc                     Disable documentation checking

        --gte FILE,THRESHOLD         If FILE contains a number, verify it is >= to THRESHOLD

        --max-violations VALUE       Max allowed violations (default: 0)
        --parallel                   Use all processors. Slower on small projects, faster on large.

    -v, --version                    Show version
    -h, --help                       Show this message

Set default options using a .cane file:

> cat .cane
--abc-glob **/*.rb
> cane

It works exactly the same as specifying the options on the command-line. Command-line arguments will override arguments specified in the .cane file.

Integrating with Rake

  require 'cane/rake_task'

  desc "Run cane to check quality metrics" do |cane|
    cane.abc_max = 10
    cane.add_threshold 'coverage/covered_percent', :>=, 99
    cane.no_style = true
    cane.abc_exclude = %w(Foo::Bar.some_method)

  task :default => :quality
rescue LoadError
  warn "cane not available, quality task not provided."

Rescuing LoadError is a good idea, since rake -T failing is totally frustrating.

Adding to a legacy project

Cane can be configured to still pass in the presence of a set number of violations using the --max-violations option. This is ideal for retrofitting on to an existing application that may already have many violations. By setting the maximum to the current number, no immediate changes will be required to your existing code base, but you will be protected from things getting worse.

You may also consider beginning with high thresholds and ratcheting them down over time, or defining exclusions for specific troublesome violations (not recommended).

Integrating with SimpleCov

Any value in a file can be used as a threshold:

> echo "89" > coverage/covered_percent
> cane --gte 'coverage/covered_percent,90'

Quality threshold crossed

  coverage/covered_percent is 89, should be >= 90

You can use a SimpleCov formatter to create the required file:

class SimpleCov::Formatter::QualityFormatter
  def format(result)"coverage/covered_percent", "w") do |f|
      f.puts result.source_files.covered_percent.to_f

SimpleCov.formatter = SimpleCov::Formatter::QualityFormatter

Implementing your own checks

Checks must implement:

  • A class level options method that returns a hash of available options. This will be included in help output if the check is added before --help. If your check does not require any configuration, return an empty hash.
  • A one argument constructor, into which will be passed the options specified for your check.
  • A violations method that returns an array of violations.

See existing checks for guidance. Create your check in a new file:

# unhappy.rb
class UnhappyCheck <
  def self.options
      unhappy_file: ["File to check", default: [nil]]

  def violations
      description: "Files are unhappy",
      file:        opts.fetch(:unhappy_file),
      label:       ":("

Include your check either using command-line options:

cane -r unhappy.rb --check UnhappyCheck --unhappy-file myfile

Or in your rake task:

require 'unhappy' do |c|
  c.use UnhappyCheck, unhappy_file: 'myfile'


Writing class level documentation

Classes are commonly the first entry point into a code base, often for an oncall engineer responding to an exception, so provide enough information to orient first-time readers.

A good class level comment should answer the following:

  • Why does this class exist?
  • How does it fit in to the larger system?
  • Explanation of any domain-specific terms.

If you have specific documentation elsewhere (say, in the README or a wiki), a link to that suffices.

If the class is a known entry point, such as a regular background job that can potentially fail, then also provide enough context that it can be efficently dealt with. In the background job case:

  • Should it be retried?
  • What if it failed 5 days ago and we're only looking at it now?
  • Who cares that this job failed?


Requires MRI 1.9, since it depends on the ripper library to calculate complexity metrics. This only applies to the Ruby used to run Cane, not the project it is being run against. In other words, you can run Cane against your 1.8 or JRuby project.


Ask questions on Stack Overflow. We keep an eye on new cane questions.


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